FIG. 5 is a configurational diagram of a storage system of the prior art which is able to charge a battery module 912 containing secondary batteries using power outputted from a solar cell 911. A switch circuit 913 is provided between the solar cell 911 and the battery module 912. A controller 914 turns the switch circuit 913 ON or OFF, and this starts or stops the charging of the battery module 912 with power outputted from the solar cell 911. The power stored in the battery module 912 may be discharged to a load not shown in FIG. 5.
In this type of system, a protection circuit 915 is often provided to protect the battery module 912 against overcharging (see, for example, Patent Document 1 below). The protection circuit 915 outputs a force OFF signal when the voltage V1 outputted to the battery module 912 (voltage applied to the battery module 912) rises above a predetermined first reference voltage (upper limit voltage) VH. When a force OFF signal is outputted from the protection circuit 915, the switch circuit 913 is always turned OFF regardless of the signals being outputted from the controller 914 to the switch circuit 913. Once a force OFF signal has been outputted, the protection circuit 915 keeps outputting the force OFF signal to reduce the output voltage V1 to below a second reference voltage VL that is lower than the first reference voltage VH, and stops outputting the force OFF signal when the output voltage V1 falls below the second reference voltage VL. When there is a single reference voltage (threshold voltage), the output voltage V1 is near the threshold voltage, and the output voltage V1 moves back and forth (in minor fluctuations) around the threshold voltage due to noise, the output of the force OFF signal also fluctuates (between outputting the force OFF signal and not outputting the force OFF signal). In order to stop this, a first reference value VH and a second reference value VL are established, and a hysteresis characteristic is provided to the start and stop of the force OFF signal output from the protection circuit 915. The voltage range between the first reference value VH and the second reference value VL is called the dead zone.
When output of the force OFF signal has been stopped and a charge ON command signal has been outputted from the controller 914, the switch circuit 913 is switched from OFF to ON. This raises the voltage outputted from the solar cell 911 above the first reference voltage VH so that the battery module 912 can be charged to the first reference voltage VH.
The following is an explanation of the charging operation performed in the storage system of FIG. 5 with reference to FIG. 6. Here, it is assumed that output voltage V1 is lower than the second reference voltage VL when the switch circuit 913 is turned OFF prior to timing t1. According to this assumption, when the switch circuit 913 is turned from OFF to ON by the controller 914 supplying a charge ON command signal to the switch circuit 913 at timing t1, the voltage V1 rises sharply until voltage V1 is at a level near the output voltage of the solar cell 911. As a result, the voltage V1 temporarily exceeds the first reference voltage VH. This causes the protection circuit 915 to forcibly turn off the switch circuit 913 at timing t2.
After being forcibly turned OFF, the voltage V1 tries to fall to voltage level prior to timing t1, that is, to a voltage level lower than the second reference voltage VL. The protection circuit 915 releases the forced OFF state after timing t2 at timing t3, and the switch circuit 913 is again switched ON and OFF. Because the voltage outputted by the solar cell 911 at timing t3 is lower than the voltage at timing t1 due to the current outputted from the solar cell 911 immediately after timing t1, the voltage V1 does not exceed the first reference voltage VH after timing t3, and the transitional voltage fluctuations converge at a level below the second reference voltage VL. As a result, the switch circuit 913 remains ON after timing t3, and the desired charging can be performed.